Although rotary propellers are by far the most common propulsion systems used in power boats, rotary propellers do present a number of problems and have a number of disadvantages. Generally, only 35 to 50 percent of the power applied to the propeller goes into boat motion, with the rest being lost to friction, wake-forming and hub and tip vortices. Only the propeller blade contributes to thrust, and then through only a portion of each revolution of the blade, and the other underwater portions of the propeller propulsion system including propeller hub, propeller shaft, and shaft support legs, only cause drag. Rotary propellers also produce a torque reaction which adversely affects the trim and steering of the boat. In addition, rotary propellers require a critical dynamic balance which is expensive to obtain and maintain; are subject to cavitation; are easily fouled by ropes, seaweed and the like, and are subject to electrolytic corrosion because of their proximity with other underwater metal parts such as the shaft, shaft bearing, etc. of different metal composition. Further, rotary propellers require expensive reverse gearing to enable reversing of the propeller drive and the rotary propellers are even less efficient when operated in reverse because of underwater structures that are in the path of the high velocity reverse flow from the propeller.
Boat propulsion apparatus have also been proposed for example as shown in U.S. Pat. Nos. 35,451; 659,858 and 1,937,907, in which an upright propelling blade is supported for swinging movement about a vertical axis on the rear end of support arms, the forward ends of which support arms are mounted on a boat for swinging movement about an upright axis so that the blade is moved with a generally fish tail type motion at the rear of the boat. In U.S. Pat. Nos. 35,451 and 1,937,907 a power operated mechanism is provided for swinging the support arms about the upright axis and the blades are supported for free swinging movement at the rear ends of the arms between angularly spaced blade stops. Such devices do not have any positive control of the blade with angle throughout the cycle of operation. In U.S. Pat. No. 659,858, a drive is provided that extends through the support arms to the blades for swinging the blades about an upright axis relative to the rear ends of the control arms in timed relation with the swinging of the control arms about an upright axis adjacent their forward ends. The blade drive mechanism in this patent must extend through the horizontally swingable arms and is therefore relatively complex and expensive to produce and maintain. In addition, the horizontally swinging arms of this patent do not contribute to the forward thrust of the propulsion apparatus and they must be relatively large to house the blade drive mechanism so that the arms provide relatively large underwater surfaces that increase drag and reduce hydrodynamic efficiency.